III-V group compound semiconductor material, such as gallium nitride based semiconductor material, gallium arsenide based semiconductor material and indium phosphide based semiconductor material, has large band gap difference, and thus is used to form various heterojunction structures. In such heterojunction structures, quantum wells and two-dimensional electron gas with high concentration can be generated near heterojunction interfaces, such two-dimensional electron gas is trapped in the quantum wells, and thus carriers and ionized impurities are separated in space, thereby reducing Coulomb force of the ionized impurities against the carriers, eliminating effect of ionizing scattering centers, thus greatly improving mobility of the carriers. Therefore, III-V group compound semiconductor material has excellent electrical characteristics. Based on the characteristics of the III-V group compound semiconductor material, High Electron Mobility Transistors (HEMTs) using III-V group compound are formed, which have the characteristics of high mobility, high carrier concentration, high frequency, high temperature, high pressure and high power, so that they can be widely used in the fields of microwave, millimeter wave and radar systems and become one of the hot research topics in the field of semiconductor devices.
A HEMT device is a kind of plane-channel field effect transistors. In an HEMT device, most electric field lines are congregated at an edge of a gate electrode which is adjacent to a drain electrode, thus an electric field peak is generated. When a voltage applied between the gate electrode and the drain electrode increases, an electric field intensity at the edge will increase promptly, resulting in increase of leakage current of the gate electrode. Thus the device is likely to fail due to avalanche breakdown. Since a bearable pressure of the device is integration of an electric field intensity between the gate electrode and the drain electrode, compared with an electric field with uniform distribution, the sharper the electric field peak at the edge of the gate electrode is, the smaller the breakdown voltage bearable by the device is. The electric field peak at the edge of the gate electrode adversely affects use of the advantages of high breakdown voltage and high power, and increase of leakage current of the gate electrode deteriorates reliability of the device.
In order to improve the breakdown voltage of the HEMT device and therefore take full advantage of high output power of the HEMT device, approaches of reducing the electric field peak at the edge of the gate electrode using a field plate structure have been developed. As shown in FIG. 1, with such a structure, an area of a depletion region is increased by using a field plate, the bearable voltage in the depletion region is improved, and thus the breakdown voltage of the device is improved. In addition, with such a structure, distribution of the electric field lines in the depletion region of a barrier layer is modulated by using the field plate, the leakage current of the gate electrode is reduced.
Although use of source field plates may increase the breakdown voltage of devices, it increases gate-source capacitance Cgs of devices, which deteriorates frequency characteristics of devices, i.e., reduces fT and fMAX. In order to reduce Cgs, as shown in FIGS. 2 and 3, an area of a portion of the source field plate overlapping the gate electrode can be reduced. However, this approach will delay frequency response of devices and affect frequency characteristics of devices. Accordingly, there is a need for semiconductor devices and methods of manufacturing the same that do not affect the frequency characteristics of devices and take full advantage of the source field plates.